1. Field of the Invention
The present invention generally relates to an image sensor, and more particularly to a binning circuit and method for a multiple-vertical-pixels-sharing image sensor.
2. Description of the Prior Art
Semiconductor based image sensors, such as charge-coupled devices (CCDs) or complementary metal-oxide-semiconductor (CMOS) sensors, are widely used, for example, in cameras or camcorders, to convert images of visible light into electronic signals that can then be stored, transmitted or displayed.
Some of the pixels (or photodiodes) in the image sensor can commonly share a circuit to reduce the overall size. The sharing structure may either be arranged in a vertical manner in which the pixels in the same column are sharing common circuitry, or in a horizontal manner in which the pixels in the same row are sharing common circuitry. FIG. 1 shows a conventional multiple-vertical-pixels-sharing image sensor 10, in which multiple groups (group A, group B, etc.) of pixels share common circuits respectively arranged in a vertical sharing manner.
As more pixels (or photodiodes) are manufactured in an image sensor, the area and thus the associated intensity of each pixel become smaller. Accordingly, the signals from two or more pixels are sometimes added up (commonly called “binning”) to enhance the intensity so as to achieve better signal-to-noise ratio (SNR).
Nevertheless, binning signals across different sharing groups in a conventional vertical sharing image sensor become difficult without using additional circuitry. For example, in FIG. 1, binning inside the group A may be easily achieved by turning on associated transfer gates (tx_A_n, n=1, 2, etc.) at the same time. However, turning on the transfer gates (tx_A_n, tx_B_n, n=1, 2, etc.) from both the group A and the group B in the same (vertical) column at the same time will catastrophically result in signal collision between/among the outputs of the groups A and B at the column node (col). As a result, the conventional multiple-vertical-pixels-sharing image sensor disadvantageously has a limited number of signals to be binned.
For the reason that a conventional image sensor, such as the multiple-vertical-pixels-sharing image sensor shown in FIG. 1, could not effectively accommodate the signal binning, a need has thus arisen to propose a novel binning circuit and method for the image sensor, particularly a multiple-vertical-pixels-sharing image sensor, for binning signals across different sharing groups.